The present invention relates to color measurements, and in particular, to the measurement and control of the color of low opacity sheet material during production.
Color in sheet materials of opacity less than 100%, such as paper, is often defined to be the color of a sufficient number of layers of that material such that the removal of a single layer does not change the color. On-line color measurement during the manufacture of these materials for the purpose of production control is, by necessity, performed on the single layer being made. The color at this point is different from that of the stack of material because of the low opacity, i.e., part of the source illumination passes through the sheet and is either lost or reflected by a material of different color. To reduce or eliminate this difference in color, the sheet being measured is often backed with a reflecting surface. If this surface has the same color as the sheet being manufactured and the interface between the sheet and reflector causes no optical perturbation, the measured color will be the same as that of the stack. This is because the backing reflector replaces the stack and is optically identical to it.
In practice, this is a difficult measurement condition to achieve because the color of the reflector is rarely the same color as the sheet, and because the transparent interface between the sheet and reflector is difficult to maintain. When the color of the reflector differs from that of the sheet, the usual condition, the color of the sheet will differ from that of an infinite stack by an amount that is a function of the difference in color between the sheet and the reflector and also of the opacity of the sheet. The difference in color is not a serious problem for sheets of constant opacity because the error it produces will be constant. Therefore, process control information can still be obtained by measuring the color of the sheet backed by the deviant reflector when an infinite stack of sheets is at the proper color. Though this measurement will be different from that of the infinite stack, controlling the process to produce this single sheet color will also produce the correct stack color.
If the opacity of the sheet varies, a difficulty arises because the fraction of the reflected energy from the backing reflector will vary and, as this reflector is a different color from the sheet, the total measured energy from the sheet will vary, i.e., the measured color will vary as a function of opacity. If process control is effected based on this measurement, the controller will attempt to produce a single sheet of the new opacity whose color is the same as the original target. A stack of this material will not have the same color as a stack of the original material.
Color can be defined as a three dimensional unit representing the response of the CIE Standard Observer to any spectral radiant energy distribution. This empirical triad of sensitivities has been constructed to simulate the perceptual response of an average human observer. As applied to reflecting objects, color can be calculated by measuring the reflectance at each wavelength and integrating the product of this reflectance, the illuminance of interest, and the Standard Observer sensitivity over the visual spectrum.
Colorant is a material added to a sheet for the purpose of adjusting its triad a* and/or b* values. This will usually also reduce its L* value. However, this is not the primary purpose. A material (such as carbon black) that adjusts L* only without an affect on chromaticity is considered an opacifier.
The color "white" can be roughly defined as that color measured in response to a spectral energy distribution of uniform intensity. More precisely, white is any color whose CIE tristimulus triad is L*=any number greater than 0, a*=0, and b*=0 where a* represents the red-green axis, b* represents the yellow-blue axis, and L* represents the lightness or overall reflectivity of the material relative to a reference material such as freshly prepared Barium Sulfate. Because human color perception has low sensitivity at several spectral areas, for example at 400-410 nm, deviations from a "flat" spectrum at those points has little effect on color. Further, a wide range of colors with small a* and b* values is commonly referred to as white. For the application of the present invention, an a* or b* value of +/-5.0 may be considered to be white. Exceeding this range will result in a gradual degradation of its usefulness.
The term "opacity" as used here refers to the ratio of the reflectance of a sheet with no reflecting background (black) to the reflectance of that sheet with a reflecting background. The TAPPI specification calls for a background of 89% reflectance.
The term "stack" is used here to mean a sufficient number of layers of a sheet material such that removal of one layer does not affect the reflectance factor of the surface layer. This is often called an infinite stack.